Stabilization of synthetic rubber



Patented Feb. 26, 1952 STABILIZATION OF SYNTHETIC RUBBER Byron A. Hunter, Seymour, Conn assignor to United States Rubber Company, New York, N. Y., a corporation of New Jersey No Drawing. Application March 17, 1948, Serial No. 15,512

This invention relates to the preservation of i synthetic rubber-like (elastomeric) materials,

and more particularly those which are derived.

from the modified 1,3-diene polymers which are formed from polymerization of conjugated dienes with copolymerizable vinyl compounds such as styrene, acrylonitrile, etc., and which include especially those butadienoid polymers known to the trade as Buna S (GR-Si which are rubbery copolymers of 1,3-butadiene and styrene; and the rubbery copolymers of 1,3 butadiene and acrylonitrile known under the trade names Buna N (GR-A), Chemigum, Hycar OR, etc. i

It is known (U. S. Patent No. 2,419,354) that triaryl esters of phosphorus acid are excellent stabilizers for GR-S and GRr-A. rubbers, and more so than the trialkyl esters of phosphorus acid. Further, it can be stated that the normal triaryl hydrocarbon and trialkyl hydrocarbon esters of phosphoric acid involving pentavalent phosphorus offer no worthwhile comparison with the aforesaid excellent stabilizers. In view of the relative inactivity of said pentavalent triaryl and trialkyl esters, it is surprising that stabilizers of such unusual merit are obtained when the present monoalkyl and dialkyl esters of phosphoric acid are used. The same can be said for the herein listed esters of the polyphosphoric acids in which 2 to 4 pentavalent phosphorus atoms occur.

Accordingly, the present invention is concerned with the use, as stabilizers for the synthetic rubber, of aliphatic hydrocarbon esters of phosphoric acids containing from 1 to 4 pentavalent phosphorus atoms and in which at least one of the acid hydrogen atoms of the acid remains unsubstituted, the stabilizer being used in amounts efiective to stabilize the synthetic rubber.

The aralkyl radicals such as benzyl, since they. are functionally aliphatic like alkyls per se, are included herein under the alkyls.

Examples of compounds can be used to stabilize the synthetic rubbers are:

Methyl diacid phosphate Dimethyl acid phosphate Ethyl diacid phosphate Diethyl acid phosphate Propyl diacid phosphate Dipropyl acid phosphate Butyl diacid phosphate Dibutyl acid phosphate Amyl diacid phosphate Diamyl acid phosphate Octyl diacid phosphate Dodecyl diacid phosphate Didodecyl acid phosphate Ethyl octyl acid phosphate Ethyl isoamyl acid phosphate 9 Claims. (CL 260 45.7)

Ethyl capryl acid phosphate Dimethyl acid pyrophosphatc Diethyl acid pyrophosphate Dl-isopropyl acid pyrophosphate Di-n-propyl acid pyrophosphate Di-n-butyl acid pyrophosphate It is to be seen that the invention is not limited to acid esters of ortho phosphoric acid but also includes the acid esters of higher phosphoric acids such as pyrophosphoric acid, tfipolyphosphoric acid, tetrapolyphosphoric acid, etc. In general, the eifective compounds include all the alkyl acid esters of pentavalent acids of phosphorus.

It can be stated, in general, that rubber solubility is promoted in proportion to increase in the number of hydrogen atoms of the hydroxyl groups that are replaced by the aliphatic hydrocarbon radicals. Also, that water insolubility is promoted in proportion to increase in the number of alkyl groups that are present. Further, that those acid esters derived from the polyphosphoric acids, e. g., pyrophosphoric, tripolyphosphoric, etc., are less water-soluble and more rubber-soluble than the acid esters from the phosphoric acid (H3PO4).

The chemicals of this invention can be made by any method known to the art. A convenient method for preparing commercial mixtures of monoand diacid esters of phosphoric acid, for example, consists in heating an alcohol with phosphorus pentoxide. Varying proportions of the monoand diacid ester will be obtained when varying molecular proportions of alcohols are used. Another method of preparing compounds useful for the purposes of the invention consists in reacting phosphorus oxychloride with one or two molecular proportions of an alcohol and hydrolyzing the resulting product to the desired acid ester. Still another method of preparing 3 stabilizers of this invention consists of heating a trialkyl phosphate with a calculated amount of a strong base, whereupon salts of dialkyl or monoalkyl phosphates are obtained. The free.

acid esters are formed upon treatment of the salts with mineral acids. Acid pyrophosphoric acid esters can be obtained by reacting an alcohol or a sulfuric acid ester of an alcohol with pyrophosphoric acid or a water-soluble pyrophosphate as described in U. S. Patent 2,053,653.

Example 1 Butadiene-styrene rubbery copolymer (GR-S) was treated with the following alkyl acid esters of pentavalent acids of phosphorus in the proportion of 1.5 parts stabilizer per 100 parts polymer: Diamyl acid phosphate, amyl diacid phosphate, butyl diacid phosphate, octyl diacid phosphate, capryl diacid phosphate, ethyl octyl acid phosphate, ethyl isoamyl acid phosphate, ethyl capryl acid phosphate, dodecyl acid phosphate, mixture of monoand dibutyl (acid) phosphates, di-isoamyl acid pyrophosphate,pentaisoamyl acid tripolyphosphate, and. tested along with a control stock.

The chemicals were added to the unstabilized synthetic rubber polymer on a rubber mill in the usual manner. Samples of the stocks were placed in an oven at 212 F. for 24 hours and were then cooled and examined. The surface of the control stock, containing no added stabilizer chemical, had resinifled and cracked easily on bending, whereas the other stocks were still in good condition and showed no evidence of deterioration. All samples were replaced in the oven and examined daily for a period of days. The control stock became increasingly harder and more brittle, and finally was no longer rubber-like. The other stocks were all still in good condition after this treatment.

Example 2 In order to demonstrate the non-discoloring and non-staining properties of the stabilizers of this invention a synthetic rubber master batch was made up according to the following recipe:

Unstabilized butadiene-styrene rubbery Portions of thi white stock were mixed on a rubber mill in the usual manner respectively with 4 v a number of the stabilizing organic phosphites in the ratio of 1.5 parts per parts rubber, said phosphites being diamyl acid phosphate, amyl diacld phosphate, butyl diacid phosphate, octyl diacid phosphate, capryl diacid phosphate, ethyl octyl acid phosphate, ethyl isoamyl acid Phosphate, ethyl capryl acid phosphate, docecyl acid phosphate, mixture of monoand dibutyl acid phosphates, di-isoamyl acid pyrophosphate, pentaisoamyl acid tripolyphosphat'e.

A control stock containing none of the stabilizing phosphites was cured along with the other stocks. A cure of 90'minutes at 45 lbs. steam was employed. Strips were cut from each stock and individual pieces were treated in one of the following ways:

(a) Dipped in a nitro cellulose base white lacquer.

(b) Dipped in a synthetic resin base white lacquer.

(c) A piece of white cotton cloth was placed in contact with the sample.

(d) Strips of the stock itself were mounted for exposure.

- All samples were exposed to the sun for 15 days.

The lacquered strips showed little or no discoloration after the treatment and no differences could be observed between the samples from the stocks containing the alkyl acid phosphates, and the samples which contained no stabilizer. Similarly, the cloth samples showed no staining nor discoloration. The exposed pieces of synthetic rubber stocks containing the chemicals of the invention remained comparatively free from discoloration and were no more discolored than the blank. These tests shows that the chemicals of this invention confer excellent stabilizing properties on synthetic rubber without imparting staining or discoloring properties.

Example 3 Hours at Chemical 212 F. to

Resiniiy Control 96 Pentaisoamyl acid tripolyphosphate 168 Di-isoamyl acid pyrophosphate The chemicals of this invention are particularly valuable as stabilizers for 1,3-butadienestyrene copolymer rubbers, and 1j3-butadieneacrylonitrile copolymer rubbers.

The stabilizers may be added by mixing with the dry synthetic rubber polymer on a rubber mill or they may be added to the synthetic rubber latex prior to flocculation. Another method of addition consists of adding the chemicals in the form of their water-soluble salts to the latex, followed by acidification and flocculation. For addition to the latex prior to flocculation, alkyl acid esters of pentavalent acids of phosphorus are chosen which have low water solubility so that the stabilizer is not removed from the synthetic rubber by the normal washing operations. Examples of compounds which have been successfully incorporated into butadiene-styrene copoly- 75 mer by addition of their soluble sodium salts to the latex are dl-isoamyl acid pyrophosphate and mixed with the latex in the form of their soluble I salts and upon acidification and flocculation of the latex very efiective and even distribution of the stabilizer throughout the rubber polymer is assured.

The deterioration inhibitors of this invention may be used alone in the synthetic rubber composition or may be mixed with other preservatives, fillers, accelerators, vulcanizing agents, or compounding agents of any sort known to the art. The components of the synthetic rubber composition may be used in various ratios, and it is not intended to limit the invention to the amounts of the ingredients indicated in the examples given.

The amount of the age resistor may vary, economical proportions ranging from 0.05 part to 5.0 parts by weight based on 100 parts by weight of the elastomer base.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. A stabilized composition of an elastomeric polymerization product of butadiene and a copolymerizable vinyl monomer selected from the class consisting of styrene and acrylonitrile, containing, by weight, based on the said polymeriza- I;

tion product, from 0.05 to 5 percent of a stabilizer consisting of an aliphatic hydrocarbon acid ester of a phosphoric acid containing from 1 to 4 pentavalent phosphorus atoms.

2. An elastomeric 1,3-butadiene-styrene copolymer composition containing, by weight, based on said copolymer, from 0.05 to 5 percent of a stabilizer consisting of an alkyl hydrocarbon acid ester of a phosphoric acid containing from 1 to 4 pentavalent phosphorus atoms.

3. A stabilized composition of an elastomeric polymerization product of butadiene and a copolymerlzable vinyl monomer selected from the class consisting of styrene and acrylonitrile, containing, by weight, based on the said polymerization product, from 0.05 to 5 percent of a stabilizer consisting of an alkyl acid ester of a polyphosphoric acid.

4. A stabilized composition oi an elastomeric polymerization product of butadiene and a copolymerizable vinyl monomer selected from the class consisting of styrene and acrylonitrile, containing, by weight, based on the said polymerization product, from 0.05 to 5 percent of a stabilizer consisting of an alkyl acid ester of pyrophosphoric acid.

5. A stabilized composition of an elastomeric polymerization product of butadiene and a copolymerizable vinyl monomer selected from the class consisting of styrene and acrylonitrile, containing, by weight, based on the said polymerization product, from 0.05 to 5 percent of a stabilizer consisting of an alkyl acid ester of tripolyphosphoric acid.

6. An elastomeric 1,3-butadiene-styi'ene c0- polymer composition containing, by weight, based on said copolymer, from 0.05 to 5 percent of a stabilizer consisting of di-isoamyl acid pyrophosphate.

7. An elastomeric 1,3-butadiene-styrene copolymer composition containing, by weight, based on said copolymer, from 0.05 to 5 percent of a stabilizer consisting of pentaisoamyl acid tripolyphosphate.

8. An elastomeric 1,3-butadiene-styrene copolymer composition containing, by weight, based on said copolymer, from 0.05 to 5 percent of a stabilizer consisting of a mixture of monobutyl and dibutyl acid phosphates.

9. A method of preserving an elastomeric polymerization product of butadiene and a copolymerizable vinyl monomer selected from the class consisting of styrene and acrylonitrile which comprises admixing therewith, by weight, based on the said polymerization product, from 0.05 to 5 percent of a stabilizer consisting of an aliphatic hydrocarbon acid ester of a phosphoric acid containing from 1 to 4 pentavalent phosphorus atoms.

BYRON A. HUNTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. A STABILIZED COMPOSITION OF AN ELASTOMERIC POLYMERIZATION PRODUCT OF BUTADIENE AND A COPOLYMERIZABLE VINYL MONOMER SELECTED FROM THE CLASS CONSISTING OF STYRENE AND ACRYLONITRILE, CONTAINING, BY WEIGHT, BASED ON THE SAID POLYMERIZATION PRODUCT, FROM 0.05 TO 5 PERCENT OF A STABILIZER CONSISTING OF AN ALIPHATIC HYDROCARBON ACID ESTER OF A PHOSPHORIC ACID CONTAINING FROM 1 TO 4 PENTAVALENT PHOSPHORUS ATOMS. 